Regulation of Lipid Metabolism by Cyclic Nucleotides

Overview

The intracellular concentration of cyclic AMP regulates triglyceride breakdown in adipocytes. However, it remains to be established whether hormones which activate lipolysis exert their effects solely through cyclic AMP. Under appropriate conditions, all agents which increase lipolysis in adipocytes increase cyclic AMP formation. Catecholamines and other activators of adipocyte lipolysis also increase the activity of adenylate cyclase, protein kinase and triacylglycerol lipase. Cholera toxin, after a lag period of 30–90 min, increases cyclic AMP accumulation in adipocytes and accelerates triglyceride breakdown. Cholera toxin inhibits the guanosine triphosphatase involved in conversion of active to inactive adenylate cyclase through NAD ribosylation of a plasma membrane protein. The addition of adenosine deaminase to rat adipocytes rapidly activates adenylate cyclase by removing membrane-bound adenosine which exerts an inhibitory constraint on basal adenylate cyclase activity. Thyroid hormones regulate adenylate cyclase activity of adipocytes by affecting the coupling of the hormone-receptor complexes to adenylate cyclase. Growth hormone also activates adenylate cyclase through a process involving synthesis of a protein(s).

Methyl xanthines and other inhibitors of cyclic AMP phosphodiesterase increase lipolysis. However, there are no hormones whose effects on lipolysis can be attributed to regulation of cyclic AMP phosphodiesterase. The effects of methyl xanthines on cyclic AMP accumulation in rat adipocytes may be due primarily to antagonism of adenosine inhibition of adenylate cyclase. Insulin activates cyclic AMP phosphodiesterase activity of rat adipocytes; it is unlikely that this accounts for the anti-lipolytic action of insulin. Similarly, the lipolytic action of glucocorticoids does not appear to involve regulation of cyclic AMP metabolism. There is even evidence that agents such as ACTH and catecholamines may activate some process in addition to adenylate cyclase which contributes to their activation of lipolysis. One possibility is hormonal regulation of the availability of triglyceride stores in the central triglyceride droplet of adipocytes to the triacylglycerol lipase in the cytosol.

₂-Adrenergic agonists inhibit hormone activated adenylate cyclase activity of adipocytes from hamsters and man. This appears to be a direct effect not mediated through calcium. There is an α ₁-adrenergic effect in rat adipocytes which results in an increase in cytosol calcium. The increase in phos-phatidylinositol turnover seen with α-adrenergic agonists is exclusively an α ₁-effect and may be involved in some unknown fashion with the release of bound intracellular calcium and entry of extracellular calcium. Alterations in the level of cytosol calcium have little effect on lipolysis; but an elevation of cytosol calcium inactivates glycogen synthase and activates glycogen phosphorylase. Insulin activates glycogen synthase in adipocytes but its action does not appear to involve either cytosol calcium, cyclic AMP, cyclic GMP, or H₂O₂. Insulin probably regulates mitochondrial pyruvate dehydrogenase and glycogen synthase through generation of an unknown second messenger. An attractive hypothesis is that the interaction of insulin with plasma membrane receptors results in activation of a protease which forms a polypeptide messenger.

The regulation of fatty acid synthesis by agents altering cyclic AMP is well recognized. Recent evidence supports the hypothesis that the key regulatory enzymes are subject to cyclic AMP dependent phosphorylation through protein kinase. Hormones activating triglyceride breakdown inhibit fatty acid synthesis; this is another example of reciprocal metabolic regulation.